A milestone for the Visible Tunable Filter

June 2017

Over the coming months, the world’s largest Fabry-Pérot interferometer (FPI) will be assembled and tested at the Kiepenheuer Institute for Solar Physics. Among its main components are two high-purity quartz glass disks, each coated on one side with a highly reflective layer. The coated surface has a diameter of 28 cm and its mean evenness does not exceed the incredible value of 0.000 000 001 m. By way of comparison, if lake Titisee in the Black Forest had this evenness, this would imply that the height of its waves remains below 5 micrometers (1/10 of a hair). On a smaller scale, the roughness of the coated surface is even 10 times better, reaching the size of individual atoms.

Picture of the upper disk of the FPI during the optical measurement process following the coating of the disk at Laboratoire des Matériaux Avancés (LMA) in Lyon. In the final assembly, the FPI will be mounted horizontally. It shows the highly-reflective coating on the future “inner surface” of the FPI. The three recesses will hold the three piezo actuators necessary for adjusting the distance between the disks in the nm range. The treatment of the glass substrate and the superpolishing of the disk were performed by Zygo-Ametek in Richmond, California. Disk coating was done at LMA using an ion source.

A few days ago, the first completely coated disk arrived at KIS where it will be watched over carefully. As soon as the second disk is delivered (due in August 2017), the team will start to work on the complex assembly including the piezo actuators and high-precision optical encoders.

This FPI is the centrepiece of the „Visible Tunable Filter“ (VTF) currently being developed at KIS. The VTF will be used as focal instrument at the US-based Daniel K. Inouye Solar Telescope. This telescope is currently under construction on the island of Maui (Hawaii). Once completed, it will be the by far the world’s largest solar telescop.